The present invention generally relates to capacitors, and more particularly to a thin electrolytic capacitor suitable for use in an implantable medical device such as an implantable cardiac defibrillator (ICD) and wherein the capacitor encasement remains electrically neutral.
ICDs are devices that are typically implanted in a patient""s chest to treat very fast, and potentially lethal, cardiac arrhythmias. These devices continuously monitor the heart""s electrical signals and sense if, for example, the heart is beating dangerously fast. If this condition is detected, the ICD can deliver one or more electric shocks, within about five to ten seconds, to return the heart to a normal heart rhythm. These defibrillation electric shocks may range from a few micro-joules to very powerful shocks of approximately twenty-five joules to forty joules.
Early generations of ICDs utilized high-voltage, cylindrical capacitors to generate and deliver defibrillation shocks. For example, standard wet slug tantalum capacitors generally have a cylindrically shaped conductive casing serving as the terminal for the cathode and a tantalum anode connected to a terminal lead electrically insulated from the casing. The opposite end of the casing is also typically provided with an insulator structure.
One such capacitor is shown and described in U.S. Pat. No. 5,369,547 issued on Nov. 29, 1994 and entitled xe2x80x9cCapacitorxe2x80x9d. This patent disclosed an electrolytic capacitor that includes a metal container that functions as a cathode. A porous coating, including an oxide of a metal selected from the group consisting of ruthenium, iridium, nickel, rhodium, platinum, palladium, and osmium, is disposed proximate an inside surface of the container and is in electrical communication therewith. A central anode selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium is spaced from the porous coating, and an electrolyte within the container contacts the porous coating and the anode.
U.S. Pat. No. 5,737,181 issued on Apr. 7, 1998 and entitled xe2x80x9cCapacitorxe2x80x9d describes a capacitor that includes a cathode material of the type described in the above cited patent disposed on each of two opposed conducting plates. A metal anode (also of the type described in the above cited patent) is disposed between the cathode material coating and the conducting plates.
U.S. Pat. No. 5,982,609 issued Nov. 9, 1999 and entitled xe2x80x9cCapacitorxe2x80x9d describes a capacitor that includes a cathode having a porous coating including an amorphous metal oxide of at least one metal selected from the group consisting of ruthenium, iridium, nickel, rhodium, rhenium, cobalt, tungsten, manganese, tantalum, molybdenum, lead, titanium, platinum, palladium, and osmium. An anode includes a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium.
While the performance of these capacitors was acceptable for defibrillator applications, efforts to optimize the mechanical characteristics of the device have been limited by the constraints imposed by the cylindrical design. In an effort to overcome this, flat electrolytic capacitors were developed. U.S. Pat. No., 5,926, 362 issued on Jul. 20, 1999 and entitled xe2x80x9cHermetically Sealed Capacitorxe2x80x9d describes a deep-drawn sealed capacitor having a generally flat, planar geometry. The capacitor includes at least one electrode provided by a metallic substrate in contact with a capacitive material. The coated substrate may be deposited on a casing side-wall or connected to a side-wall. The capacitor has a flat planar shape and utilizes a deep-drawn casing comprised of spaced apart side-walls joined at their periphery by a surrounding intermediate wall. Cathode material is typically deposited on an interior side-wall of the conductive encasement which serves as one of the capacitor terminals; e.g. the cathode. The other capacitor terminal (the anode) is isolated from the encasement by an insulator/feedthrough structure comprised of, for example, a glass-to-metal seal. It is also known to deposit cathode material on a separate substrate that is placed in electrical communication with the case. In another embodiment, the cathode substrate is insulated from the case using insulators and a separate cathode feedthrough.
A valve metal anode made from metal powder is pressed and sintered to form a porous structure, and a wire (e.g. tantalum) is imbedded into the anode during pressing to provide a terminal for joining to the feedthrough. A separator (e.g. polyolefin, a fluoropolymer, a laminated film, non-woven glass, glass fiber, porous ceramic, etc.) is provided between the anode and the cathode to prevent short circuits between the electrodes. Separator sheets are sealed either to a polymer ring that extends around the perimeter of the anode or to themselves.
A separate weld ring and polymer insulator may be utilized for thermal beam protection as well as anode immobilization. Prior to encasement welding, a separator encased anode is joined to the feedthrough wire by, for example, laser welding. This joint is internal to the capacitor. The outer metal encasement structure is comprised essentially of two symmetrical half shells that overlap and are welded at their perimeter seam to form a hermetic seal. After welding, the capacitor is filled with electrolyte through a port in the encasement.
The above described techniques present concerns relating to both device size and manufacturing complexity. The use of overlapping half-shields results in a doubling of the encasement thickness around the perimeter of the capacitor thus reducing the available interior space for the capacitor""s anode. This results in larger capacitors. Space for the anode material is further reduced by the presence of the weld ring and space insulator. In addition, manufacturing processes become more complex and therefore more costly, especially in the case of a deep-drawn encasement.
A further disadvantage of the known design involves the complexity of the anode terminal-to-feedthrough terminal weld joint. As was described, a tantalum anode lead is imbedded into the anode and is joined via laser welding to a terminal lead of the feedthrough. This is typically accomplished by forming a xe2x80x9cJxe2x80x9d or xe2x80x9cUxe2x80x9d shape with one or more of the leads, pressing the terminal end of these leads together, and laser welding the interface. In order to accomplish this, one must either perform this step prior to welding the feedthrough ferrule into the encasement or sufficient space must be provided in the capacitor anode structure to facilitate clamping and welding while the anode is in the case. This results in additional manufacturing complexity while the latter negatively impacts device size.
As stated previously, a separator material is provided on the anode and may be sealed to itself to form an envelope. The anode is typically on the order of 0.1 inch thick. As a result, the sealing operation is complex, and significant separator material typically overhangs the anode. This overhang must be accommodated in the design and typically either reduces the size of the anode or increases the size of the capacitor. Furthermore, due to the proximity of thermally sensitive separator material to the encasement, the separator is in direct contact with the cathode/encasement structure. Weld parameters must therefore be carefully selected to prevent thermal damage of the separator material. When cathode material is deposited on a separate substrate, as described above, substrate thickness further reduces the space available for anode material or increases the size of the capacitor. In a case-neutral device (i.e. the capacitor encasement forms neither the anode terminal or the cathode terminal), the additional space necessary to incorporate separate feedthrough ferrules and insulators to insulator the cathode and the anode from the case further increases the size of the capacitor.
Thus, while the development of flat electrolytic capacitors significantly reduces size and thickness, defibrillation capacitors are still the largest components in current ICDs making further downsizing a primary objective.
According to an aspect of the invention, there is provided a capacitor structure, comprising a shallow drawn encasement having first and second major sides and a peripheral wall coupled to the first and second major sides. A cathode is disposed within the encasement proximate the first and second major sides, the cathode having a cathode lead. A central anode having an anode lead is disposed within the encasement, and a bipolar, insulative feedthrough extends through the encasement through which electrical coupling may be made to the anode lead and the cathode lead.